Navigational aids are useful to assist the driver of a vehicle in locating his current position and for locating and moving to a desired destination. Typically, the navigational aid used by the driver is a conventional paper street map of a given area which is read to determine the present location of the vehicle relative to the desired location. Another navigational aid for the driver includes a transparency of a street map placed over a monitor which shows the approximate path of a vehicle. The map transparency is visually similar to the paper street map in that, for example, it shows the same detail of streets and landmarks and the same size of lettering of names or labels for the streets and landmarks. Yet another navigational aid is a video image of a map which appears on a monitor and accurately reproduces the image of a paper street map.
One problem with these prior navigational aids, either with the paper maps, the map transparencies or the map video image, is that they present the driver with more information than is necessary for navigating the vehicle. These maps are confusing and difficult to use because the driver may be able to take only short occasional glances at the map while engaged in the task of driving. For example, all streets and landmarks are depicted based on a priority scheme in which, for example, the streets are categorized and highlighted by interstate highways, state highways, major roads, access roads, local streets, etc. This detail of information, including also all the names of the streets and landmarks, is always presented to the driver even though the driver may need to read only the local streets to determine the route to his or her local destination. Alternatively, the driver may want to view only the major road network, but this may not be easily visible amid the clutter of the local streets and street names. Consequently, all the additional and unnecessary information that is on the map will be distracting for a given navigational purpose.
Furthermore, the details shown in the paper map or the map transparencies may not enable the driver to grasp quickly "the lay of the land" and get a feel for his or her location and orientation with respect to the street network and/or destination. For example, the driver may not easily perceive his current position or the current heading or direction of movement of the vehicle relative to surrounding streets or landmarks.
Also, it may be desireable to change the scale of the map display to study in detail, for example, a small geographical area or to gain perspective of a large geographical area. Paper maps and map transparencies require physically changing the map being viewed. For video images, scaling can be automatically accomplished on the monitor, but the street labels are displayed such that their size is dependent on the scale level. This is distracting, requiring the driver to adjust his vision to different sized labeling. And, if the video image is rotated to match vehicle heading, the fixed labels will create upside down writing.